This invention relates to ring laser angular rate sensors, and more particularly to an apparatus for providing antilocking biasing.
Ring laser angular rate sensors are well known in the art of inertial navigational systems. Some examples of ring laser angular rate sensors are shown and described in U.S. Pat. Nos. 3,323,411, 3,373,650, 3,390,606, 3,467,472, and 4,152,071, all of which are assigned to the assignee of the present application. Ring laser angular rate sensors comprise a pair of laser beams which travel or propagate in opposite directions along a closed-loop path or cavity. Rotation of the sensor causes the frequency of the laser beams to change and exhibit a frequency difference which is proportional to the rotation rate of the sensor. However, at low rotation rates, the laser beams tend to "lock-in" to a common frequency. The "lock-in" phenomenon is thought primarily to be caused by backscattering of the laser beams along the closed-loop path, and particularly at the reflection surfaces defining the closed-loop path.
Research and development related to the ring laser angular rate sensor has been primarily directed to apparatuses and methods to avoid, reduce, or correct for the effects of lock-in. U.S. Pat. No. 3,323,411 recognized that the backscattering energy could be neutralized by introducing back into the cavity of closed-loop path a beam of energy equal in amplitude but opposite in phase to the sum of the undesirable reflections which cause the backscattering. Since opposite phase beams neutralize each other, the controlled redirected beam could cancel out the backscatter energy and thus minimize or prevent lock-in. U.S. Pat. No. 3,697,181 by Warren M. Macek also redirects energy back into the cavity, but the redirected energy is phase and amplitude controlled in response to amplitude modulation of the countertraveling laser beams which is a function of backscattering.
U.S. Pat. No. 3,373,650 takes an altogether different approach to minimizing the effects of lock-in. The U.S. Pat. No. 3,373,650 teaches the use of a mechanical rotation or dither which rotationally oscillates the sensor back and forth. This oscillation causes the frequencies between the two countertraveling laser beams to vary in direct relationship to the rotation rate caused by the mechanical dithering. Mechanical dithering is known to reduce lock-in substantially.